This invention relates to electrically conductive, thermoplastic melt processable compositions that employ a major amount of a fluorothermoplastic polymer, and minor amounts of an electrically conductive material and a polyolefin.
Fluoropolymers are often used for their desirable properties, such as low surface tension, high thermal stability, and high resistance to chemicals, oils, and/or solvents.
Examples of fluoropolymers include copolymers of tetrafluoroethylene with one or more fluorinated monomers such as hexafluoropropylene or perfluoropropyl vinyl ether, and/or non-fluorinated monomers such as ethylene or propylene.
Often it is desirable that the fluoropolymer have a lower electrical resistance or even be electrically conductive. Fluoropolymers are typically made more electrically conductive by adding electrically conductive fillers (e.g., carbon black, carbon fibers, etc.). However, the addition of such fillers has certain negative effects on the properties of the fluoropolymers. For example, while adding conductive fillers desirably enhances the electrically conductivity of the fluoropolymer, it also often undesirably reduces the melt processability of the fluoropolymer.
The present invention provides a thermoplastic melt-processable fluoropolymer with lower electrical resistivity without increasing the level of the conductive filler. Hydrocarbon polymers, such as polyolefins, are well known in the art as electrical insulators. Surprisingly, adding a normally insulating hydrocarbon polymer to what would be a conductive composition actually improves the conductivity of the resultant mixture in this invention. The present invention also provides lower-resistivity fluoropolymers with improved processability and higher melt flow indices than known materials having comparable resistivity.
Briefly, the present invention provides a melt-processable conductive fluorothermoplastic composition comprising a blend of at least three components. The first component provides a major amount (i.e., at least 50 weight percent) of at least one melt-processable, thermoplastic fluoropolymer component. This first component has its interpolymerized units derived from (i) at least 50 weight percent (wt %) of tetrafluoroethylene (TFE), and (ii) one or more ethylenically-unsaturated monomers represented by the formulas CF2xe2x95x90CFxe2x80x94Rf, CF2xe2x95x90CFxe2x80x94Oxe2x80x94Rfxe2x80x2, and CH2xe2x95x90CR2. In the preceding formulas, Rf is a perfluoroalkyl of 1 to 8, preferably 1 to 3, carbon atoms, Rfxe2x80x2 is Rf or a perfluoroalkoxy of 1 to 8, preferably 1 to 3, carbon atoms, and R is selected from H, F, Cl, or an aliphatic group having from 1 to 8, preferably 1 to 4, carbon atoms which aliphatic group may have F or Cl substituents. The first component has less than 5 wt % of its interpolymerized units derived from vinylidene fluoride. The second component is a hydrocarbon polymer, present in the blend at a level from about 0.1 to about 10 wt %. The third component is a conductive filler, present in the blend at a level from about 1 to about 20 wt %.
The present invention also provides a method of improving volume resistivity of a melt-processable conductive fluorothermoplastic composition comprising the steps of providing at least the three components described above, and mixing the components.
The present invention also provides shaped articles incorporating a fluorothermoplastic composition as described above.
The extrudates of the present invention substantially retain properties of the fluoropolymer, such as thermal stability and/or chemical resistance. These extrudates exhibit lower resistivity than known fluorothermoplastic compositions having similar levels of conductive fillers. More specifically, the inventive compositions have a volume resistivity below about 1xc3x97104 ohm cm, more preferably below about 1xc3x97102 ohm cm.
The lower resistivity is surprisingly achieved while maintaining good melt processability or extrusion behavior. The melt flow indices of the inventive compositions do not decrease as rapidly as known materials when the level of conductive filler is increased. Thus, the extrudates of the inventive blend composition can be extruded at higher output rates and at much higher shear rates with much reduced shear stress, as compared to known fluorothermoplastic compositions having similar resistivity levels. These inventive extrudates also have good surface qualities, particularly smoothness, and are otherwise relatively free of objectionable surface melt defects.